Voltage surge and voltage transient suppressors are commonly employed between a power source and its electrical load. Such suppressors protect equipment from surges and transient spikes that can occur on a power line as a result of inductive load switching, lightning strikes, and other transient events on a power line. Surge suppressors can also prevent switching transients generated within a load from being reflected back into the power source and to other equipment.
For certain applications, the surge suppressor is required to meet the standards specified by ANSI standard C62.41 (IEEE Standard 587-1980), which, for example, requires that 1.2.times.50 .mu.s, 6000 V and 8.times.20 .mu.s, 3 kA voltage and current impulse waves, respectively, must be attenuated to less than two times the nominal peak system voltage. The suppressor should be able to dissipate the energy contained in the spike as limited by the impedance of the source, and the leakage or standby current drawn by the suppressor should be limited to 1% of the rated line current.
Metal oxide varistors (MOVs) are often employed as surge suppressors. MOVs are voltage clamping devices and are usually connected directly across a power line. An MOV does not clamp until the occurrence of a voltage transient (spike) exceeding the line voltage by a sufficient amount. As the voltage transient rises, the MOV's nonlinear impedance results in a current spike through the MOV that rises faster than the voltage across it. This produces the desired voltage clamping action.
Spark gaps are also employed in surge suppression systems. A gas tube is essentially a spark gap with the electrodes hermetically sealed in a gas-filled ceramic enclosure to lower the breakover (or breakdown) voltage. This type of device is small and inexpensive and has the capability of withstanding pulse currents of up to 20,000 A. When the gas tube breaks over, the typical arc voltage ranges from 10 to 30 V. However, the breakdown voltage varies, since it is dependent on the rise time of the applied surge. For example, the typical sparkover voltage for a presently available gas tube rated for a 460 V AC application ranges from 1100 volts for a 100 V per microsecond surge to 1500 V for a 10 kV per microsecond surge. Thus, depending on the applied transient, several microseconds may elapse before a typical gas tube arcs over, leaving the leading portion of the surge intact to be passed on to the protected equipment.
Although the gas tube diverts the majority of the surge current when it breaks over, the leading portion of the surge, the "surge remnant," can contain a considerable amount of energy and have a high voltage amplitude. To clip the surge remnant, a common practice is to insert an L-section suppression circuit in the line following the gas tube. The L-section suppression circuit includes a series impedance and a voltage clamping device, such as an MOV, connected across the power line. The series impedance is connected between the gas tube and the clamp and can simply be a resistor or an inductor, or both. The impedance must be high enough in value to guarantee gas tube breakover so that the clamp only clips and diverts the energy in the remnant, not the energy in the entire surge.
A problem associated with gas tubes is "follow-on" current, which is the current from the power source that continues flowing through the gas tube after the surge current terminates. In AC circuits, the follow-on current clears when the line current goes through zero but can reappear on the next cycle of the line current. In DC applications, a separate means for extinguishing the arc must be included in the circuit.
Further information on the surge suppression art can be found in U.S. Pat. No. 5,388,021, Feb. 7, 1995, titled "Voltage Surge Suppression Power Circuits." This patent discloses circuits that employ clusters of two or more MOVs connected in parallel for suppressing surges and transients.
Complying with the ANSI standards for line-connected equipment usually requires the use of MOVs connected from Hot to Neutral, Hot to Ground, and Neutral to Ground. Although MOVs perform reasonably well in protecting electronic equipment, they present a number of problems, including:
1. Cost: MOVs are relatively expensive and difficult to install. PA1 2. Safety: MOVs fail very violently if operated at continuous voltages above their ratings. PA1 3. Manufacturability: MOVs connected to ground make performing hipot (high potential) dielectric testing nearly impossible, since the hipot test voltage cannot be set above the rating of the MOV connected to ground.